I would recommend that you choose your battles.
When I refused a formal baptism my parents performed a secret "home baptism"when I left they baby at their house for a sleepover. It's imaginary voodoo that the child will never remember. If it makes them happy then no skin off my nose. In fact, it was a small victory for them that helped them lighten up on issues that were more important in my book. But....I would use these first few years to look into job relocation in another city. Proximity seems to encourage grandparents to impose their will on grandkids.

I'd tell him, "You've had your turn. Now it's mine. My word -- NOT yours -- is final. With all due respect -- and then some -- butt out once and for all. I will not put up with any interference or bullying from you!"

You may have to cut your parents off. Or they may cut you off. I wouldn't give in; if they want to perform a secret baptism like Violet Chartreuse mentioned, that's their silliness. (Though such did result in kidnappings of the "catholic child" in the Papal States in the 1800s.)

If they make it unbearable for you to see them, stop seeing them? Warn them you will do this and say 'I don't believe what you believe and must live my life accordingly.' If they continue, you will feel justified in not seeing them.

I worry about these sorts of battles to come in my life should I have children. My parents (also strict roman catholics) are silent now on the topic of atheism. They no longer bully me, ask me to go to church, or slip pamphlets from church into my bags. This is likely all due to time, space, and age - I just turned 30. I'm enjoying the peace (well the peace on this topic anyway). I can only imagine that it will all resurface if they have a grandchild to, "protect," from hell, the devil, and its mother's atheist ways. I guess the point is that they probably won't stop, and even if they do, something else down the road might trigger it all again - but they're family so hopefully it's something you all can work on together. Right now (IMO) your situation is all anticipation, imagination, and hope of what is to come. When they bring it up, tell them that you're just focused on your wife and the health of your family and the love you all share. Focus on what your baby will have - love, shelter, nourishment, doting grandparents, etc. I think atheism is tough for a lot of people to understand because it's seen as, "being without," but none of us are devoid of emotion or values. We, as atheists, just have to work harder at showing people what we care about. This is somewhat unfair since not all theists really embody the so-called values of their faiths, but we all have limited time and resources and labels are the easy way out.

P.S. If that doesn't work, you could always research Pascal's Wager.

P.P.S. Don't you find it a bit macabre that they're focusing on preparing for the death of a grandchild before it's even born? I always have. Not that pointing that out would help much, just a personal comment.

My step mom is the same way about me going to church. She knows that I'm a atheist and won't stop, every single Sunday, for the past 4 months. Begging me to come and waste my time. I try to just ignore it. but that's all I can do since I live with her and my father. Can't wait until I go to college this year. Run out the door.